Displacement reactions in gypsum. I

Displacement reactions between gypsum cleavage plates and alkali hydroxide solutions were investigated by means of TG, DSC, microscopy and X-ray. The nucleation and crystallization of the reaction products—Ca(OH)₂ and CaCO₃—were followed in relation to time, temperature and concentration of the MeOH solutions.     

Displacement reactions between metal and liquid sulfides

Displacement reactions between liquid sulfides and metals are studied and compared with the resultant morphologies previously observed in solid-state sulfide-metal and oxide-metal displacement reactions. In all cases the aggregate morphology was observed indicating that the rate controlling step in a tentatively assumed layered arrangement would be diffusion of atomic sulfur through the product metal. The experimentally observed morphologies for NbNi₃S₂ (1), NbFeS (1) and CuNi₃S₂ reaction couples are reported and rationalized. Detailed analysis of the liquid sulfide displacement reactions considered are not possible because of the lack of available thermodynamic and diffusivity data for the sulfide systems.     

Displacement reactions of 5-chloro-4-phenylazopyrazoles

3-Methyl or 3-phenyl-5-chloro-1-phenyl-4-phenylazopyrazole with primary amines, sodium polysulfide and sodium azide yielded amines, disulfides and 2,4-dihydropyrazolo[3,4-d]-1,2,3-triazoles respectively. 4-Amino-5-anilino-1,3-diphenylpyrazole was also prepared and characterized.     

Displacement reactions of covalently attached organosilicon monolayers on Si

The covalently attached monolayers of alkylsilanes (R(CH(3))(2)SiX) on Si undergo complete displacement by the solutions of different organosilanes (R'(CH(3))(2)SiX). By varying the reaction time, the degree of displacement can be controlled offering a convenient method for the preparation of surfaces with mixed functionalities (R and R').     

DTA determination of the anhydrite content in rock salts

The DTA method was used to determine the phase diagram of the NaCl-CaSO₄ system over the range 0.55 wt.% of CaSO₄ in air atmosphere. The effects of SiO₂, CaCO₃ and Fe₂O₃ on the melting temperatures of pure NaCl and the eutectic were determined. On the basis of the above temperatures, a method of quantitative determination of the anhydrite content in rock salts has been developed.

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Zusammenfassung DTA wurde benutzt, um in Luft im Bereich 0 bis 55,0 Gew.% CaSO₄ das Phasendiagramm des Systemes NaCl-CaSO₄ zu bestimmen. Es wurde auch der Einfluß von SiO₂, CaCO₃ und Fe₂O₃ auf die Schmelztemperatur von reinem NaCl bzw. des Eutektikums bestimmt. Auf der Grundlage obiger Temperaturen wurde eine Methode zur quantitativen Bestimmung des Anhydritgehaltes in Steinsalzen entwickelt.

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- 0–55,0 . , . , ( ) .

     

Partition coefficient of Ra in gypsum

A partition coefficient (λ) for Ra in gypsum was determined from coprecipitation experiment using oversaturation method. The λ-value derived for Ra was: λ _Ra = 0.32±0.15. Saturation state was estimated with the initial concentration of Ca and SO₄²⁻ in an experiment of the present study. Saturation index was calculated to be (0.49±0.02). This value was similar to those corresponds to analogous case where slow precipitation rate was kept in coprecipitation experiment for Sr in gypsum. Therefore, derived λ-value is thought to describe partition of Ra and Ca in solid phase and solution under an equilibrium condition without the effect of kinetic of precipitation. Determined λ-value was compared with those of other alkaline elements in sulfate minerals. The derived λ-value is smaller than that of Ra in barite and is similar to those of Sr in gypsum. This trend agrees with mechanical understanding for the size effect against partition coefficient.     

Kinetics of gypsum dehydration

A systematic study was conducted to investigate the correlation between the dehydration of gypsum and the time and temperature of calcination as well as the grain size of the starting material.It was found that the proposed formula

may be considered as a real expression of the dehydration raction throughtout the major part of α. However, the initial and final stages of dehydration revealed a slight shift due to the induction period and the residual amount of water retained by the soluble anhydrite crystal lattice, respectively. The dehydration reaction at 160°C is nearly double that at 100°C and four times that at 70°C, using one particle size. Generally, as the grain size of gypsum becomes less than about 0.13 mm an abrupt increase in the dehydration process occurs with any slight decrease in diameter. A moderate change occurs, however, as the grain size changes over 0.13 mm.     

Molecular dynamics simulation on the interaction between polymer inhibitors and anhydrite surface

Investigation on the microscopic interaction between polymer inhibitors and calcium sulfate will be helpful for understanding its scale inhibition mechanism and can provide a theoretical guidance to developing new scale inhibitors. In this work, molecular dynamics simulations with COMPASS force field have been performed to simulate the interaction between hydrolyzed polymaleic anhydride (HPMA), polyaspartic acid (PASP), polyepoxysuccinic acid (PESA), polyacrylic acid (PAA) and the (001) and (020) surfaces of anhydrite (AD) crystal with and without water. The results show that the sequence of binding energies between four polymer inhibitors and AD (001) and (020) with water is PESA > PASP > HPMA > PAA. The binding energy of the same polymer inhibitor on AD (001) is smaller than that on AD (020). Water molecules weaken the deformations of HPMA and PAA but aggravate those of PASP and PESA. Natural bond orbital (NBO) charges of the repeat units of polymer inhibitors were calculated by B3LYP/6‐31G* method. The Coulomb interaction is formed between the O atoms of polymer inhibitors and the Ca atoms of AD crystal. The system of polymer–AD is mainly contributed from the non‐bonding interaction. Polymer inhibitors do not interact directly with AD crystal, but indirectly through the interactions between inhibitor–H₂O and H₂O–AD, i.e. water molecules participate in scale inhibition of polymer inhibitors to AD crystal. Water molecules cannot be ignored when the interaction models are constructed, i.e. solvent effect cannot be ignored. Copyright © 2015 John Wiley & Sons, Ltd.     

Properties study of anhydrite(II) prepared from waste gypsums

Anhydrites(II)- CaSO₄ were prepared from two waste gypsums (PRECHEZA Perov, FOSFA Potorná). The samples of anhydrites(II) with sulphate activators were tested on spillage, beginning and the end of setting time, bending and pressure strength. Both sodium and potassium sulphates accelerated setting of anhydrite, more impressively that prepared from Potorná gypsum. The addition of Na₂SO₄ influenced the strength of this anhydrite very favourably. On the contrary K₂SO₄ influenced favourably the strength of Perov anhydrite. Results confirm the necessity to evaluate properties of such materials with respect to their origin.     

Emanation of radon from fly-ash and gypsum

The emanation of radon from fly-ash and gypsum can be estimated from a simple model. The cases of a fly-ash dump and a room panelled with gypsum plates are considered.     

Thermogravimetric evaluation of the kinetics of the gypsum-hemihydrate-soluble anhydrite transitions

Study of the gypsum-hemihydrate-soluble anhydrite transitions by thermal, X-ray and IR methods showed differences in the intensity of the ∼3493 cm⁻¹ IR absorption peak of the gypsum samples and differences in the peak ratios of the DTA curve at the gypsum-hemihydrate transition. There were also differences in the temperature and rate of the γ−β anhydrite transition. This suggests that different gypsum species occur, specially among synthetic gypsum. Fifteen equations were tested in order to find models which fitted the gypsum-hemihydrate and the hemihydrate-anhydrite transitions. No model fitted all the samples. The best fit for the gypsum-hemihydrate transition in three samples was an order of reaction equation while for the hemihydrate-anhydrite transition the best fit in four samples was a power law. Differences in crystallite characteristics appear to be one of the main reasons for the differences in kinetics between the samples. Dedicated to Professor Lisa Heller-Kallai on the occasion of her 65^th birthday     

Study of gypsum by PDSC

Calcium sulfate dihydrate has been widely characterized by both differential scanning calorimetry and thermogravimetry (TG). Two dehydration processes were reported to be partially overlapping. High resolution TG and water vapor self generated atmosphere pin-hole lid strategies were used to increase the resolution of both dehydration processes. In this study, isobaric experiments were carried out in a pressure differential scanning calorimetry cell. The approach consisted in combining the pin-hole lid with different pressures with nitrogen atmosphere. Resolution was improved at moderately low pressures. At higher pressures other processes were observed.     

Photocatalytic activation of gypsum cements

Photocatalytic activity of gypsum cements that are based on the building plaster blend and synthetic calcium sulfate and contain 2–10 wt % titanium dioxide of anatase modification was studied. The decomposition rate of the model contaminant (Rhodamine C) on the surface of a plaster blend containing 2 wt %TiO₂ becomes seven times higher under exposure to a full spectrum of a UV lamp, and 29 times higher under UV-A irradiation at λ > 320 nm. It was found that there are two competing processes (photocatalysis and direct photolysis), which affect the decomposition rate of Rhodamine C on the surface of the cements, depending on the UV irradiation spectrum.     

Infrared reflection spectra of gypsum

Polarized i.r. reflection spectra of gypsum single crystals have been measured and analysed using the Kramers—Kronig relation. The transverse and longitudinal frequencies are obtained together with the optical and dielectric constants of the crystal. The librational bands of water of crystallization are identified based on the i.r. spectra of deuterated powders and low-temperature i.r. spectra. Polarized Raman spectra have also been measured, which agree well with those reported previously.     

ESR studies of Anatolian gypsum

Electron spin resonance (ESR) was used to date the formation and most recent recrystallization of three types of gypsum samples (massive, bedded and fracture filling gypsum) from the Sakarya Formation at Eski?ehir in the middle of Turkey. The ESR spectra had the signals of Fe3+ and Mn2+ in addition to those of the G1 and G2 sensitive centers (g = 2.002 and 2.008) to artificial gamma-irradiation. ESR intensities of G1 and G2 were enhanced by gamma-ray irradiation to give equivalent doses DE) for each sample. The ESR ages derived from the annual doses of 238U, 232Th and 40K contents of the samples determined by thermal neutron activation analysis (TNAA) were between 43 +/- 18 ka (massive gypsum) and 1100 +/- 466 ka (bedded gypsum) as minimal and maximal values. The ages fall into the upper Miocene-Pliocene Epoch of the geological time scale, which agreed with the stratigraphy.     

Characterization of gypsum using TMDSC

Gypsum is a dihydrated calcium sulfate, with the composition of CaSO₄⋅2H₂O, with large application interest in ceramic industry, odontology, sulfuric acid production, cement, paints, etc. During calcination, a phase transformation is observed associated to the loss of water, leading to the formation of gypsum or anhydrite, which may present different phases. The identification of the phases is not so easy since their infrared spectra and their X-ray diffraction patterns are quite similar. Thus, in this work, temperature modulated differential scanning calorimetry (TMDSC) was used to identify the different gypsum phases, which can be recognized by their different profiles.     

Comparative studies on fire-rated and standard gypsum wallboard

The long-term goal of this research is to improve the fire resistance of gypsum wallboard (GWB). GWB consists mainly of gypsum, i.e., calcium sulfate dihydrate, CaSO₄·2H₂O. In buildings, the chemical, mechanical, and thermal properties of GWB play an important role in delaying the spread of fire. To build a fire resistant GWB, it is very important to study the thermal, mechanical, physical, and chemical properties of regular GWB and various types of fire resistant wallboards available commercially in the market. Various fire resistant GWBs have been compared and contrasted with reference to a standard wallboard in this study. Regardless of the type of wallboard, the main component is gypsum. The fire resistance property is mainly attributed to the absorption of energy related with the loss of hydrate water going from the dihydrate (CaSO₄·2H₂O) form to the hemihydrate (CaSO₄·½H₂O) and from the hemihydrate to the anhydrous form (CaSO₄) in a second decomposition. The present paper is a comparative study of commercially available standard, fire-rated Type X, and fire-rated Type C GWBs. Type X wallboards are typically reinforced with non-combustible fibers so as to protect the integrity of the wallboard during thermal shrinkage, while the Type C wallboards are incorporated with more glass fibers and an additive, usually a form of vermiculite. These Type C wallboards have a shrinkage adjusting element that expands when exposed to elevated temperature. Differential scanning calorimetry, thermogravimetric analysis, thermomechanical analysis, and powder X-ray diffraction were used to characterize and compare the materials. Various properties, such as the heat flow, mass loss, dimensional changes, morphology, and crystalline structures of the GWBs were studied using these techniques.     

Thermal behaviour of gypsum based composites

DTA/TG and X-ray investigations were carried out on different building composites in order to examine their relative hydration processes. The presence of lime, hemihydrate gypsum, ferosilicate, and some other wastes as leaner and hydrophobic additive in different proportions into composites provokes hydration reactions, leading to calcium silicate hydrate (CSH), ettringite and an intermediate phase formation with varying chemical composition of calcium, aluminum, silicon and sulfur. DTA curves indicate several transformations taking place between composites components, related to hydration of some phases. The content of ettringite component corresponds to the properties and it is used as an indicator for the possibility of industrial application. It is found out that gypsum based cementitious binders could be used as building material in the industry. The same time it is confirmed that the selected wastes could be considered as secondary raw materials.